Tweak plot layouts

- No longer label each bar in "number of sources per image" with the bar
  height and the image timestamp: this doesn't scale to large numbers of
  images, rendering the plot unreadable.

- Tweak the layout of "scatter of individual sources" such that the axis
  labels don't fall off the plot.
Merge pull request #25 from jdswinbank/plot-layout

Tweak plot layouts